The correct spelling of the word "re warned" is actually "rewarned." The prefix "re-" means to do something again, and the word "warned" means to give someone a cautionary advice. Therefore, "rewarned" means to give someone a cautionary advice again. The IPA phonetic transcription for "rewarned" is /ri:ˈwɔrnd/. It is pronounced as "ree-wawrnd" with the stress on the second syllable, and the "a" sounds like "aw".
Re warned is not a specific word with its own etymology. It appears to be a combination of the prefix re- meaning again or back and the verb warned which is the past participle form of the verb warn.